(a) Field of the Invention
This invention relates to a method for the magneto-optical measurement of currents by means of a linearly polarized light beam, the plane of polarization of which is rotated as a function of the magnitude of the current to be measured, and by means of a modulation signal which is superimposed on the measurement.
(b) Description of the Prior Art
Devices for measuring currents in high voltage conductors and for measuring large AC currents with a DC component are known in which a light beam is fed via a polarizer, a magneto-optical measuring sensor, and an analyzer to a detector followed by electronic circuitry. The plane of polarization of the light beam is subjected to a rotation corresponding to the magnitude of the current in the measuring sensor which is influenced by the magnetic field of the current to be measured. In an evaluator arranged at low voltage potential, the magnitude of the rotation is converted, in an analyzer, into a signal of corresponding intensity which can be picked up by the photo detector. The output signal of the detector is processed in the electronic circuitry.
In one known arrangement, the evaluator contains an analyzer in which the polarized light coming from the measuring sensor, preferably a laser beam, is split into two partial light beams, the planes of polarization of which are orthogonal and which change their intensity in opposite directions with the angle of rotation of the polarization of the incident beam. Each of the partial light beams is fed to a detector, preferably a semiconductor photo diode, the outputs of which are fed to a differential amplifier. The difference voltage serves as a measure for the Faraday rotation of the measurement signal. Although, in theory, noise components of the measurement signal caused by intensity variations of the light beam can be eliminated by this differential method, it is unavoidable in practise that there should be local variation of the laser beams on the semi-conductor photo diodes. Since the photo sensitivity of these diodes is location dependent, beam displacements of a few .mu.m can cause signal variations of up to several percent. In addition, such photo diodes exhibit a small spread in sensitivity from unit to unit. The detectors associated with the two partial beams therefore furnish different signal variations which cannot be eliminated by this method.
Another known method (Rogers in "Optical Methods for Measurement of Voltage and Current at High Voltage", A.I.M., Liege, Traitment des donnees-1977, page 6, paragraph 3.2(c), Intensity Distribution Noise) uses a modulated light beam which is fed via the measuring sensor and an analyzer to a single photo diode, the output signal of which is processed in an electronic circuit. The detector measures the intensity of the arriving measurement signal, which also contains the superimposed modulation signal. The demodulation takes place in the connected electronic circuitry. The intensity is influenced by the Faraday rotation in the measuring sensor as well as by the noise components mentioned. With this measuring method, both intensity components can be separated from each other and the noise component on the measurement signal is suppressed. The linearity between the useful and the measurement signal, however, is sufficient only at small angles of rotation of up to about 2.degree.. At larger angles of rotation with a correspondingly larger signal to noise ratio, a non-linearity which depends on the signal amplitude is noticeable.
It is an object of the present invention to describe a very accurate method of measurement having a measuring error of, for instance, less than 0.2%, which allows a substantially larger rotation of the plane of polarization.